16th Jun 2017, 4:22 pm | #61 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
Do you have anything to test the gain of that RCA device (re my post #15)?
B
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16th Jun 2017, 8:09 pm | #62 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
Hi,
At risk of stating known knowledge hfe gain is both temperature and current dependant , using a peak atlas pocket tester at Ic of 2.5 mA and the SAME 2N5109 (Motorolla logo but new not old stock) transistor I get : T/Deg C Gain 0 , 60 25, 73 55 , 87 Temperature is liquid with a thermocouple and the transistor part submerged so close but not precise. Pete Last edited by G4_Pete; 16th Jun 2017 at 8:30 pm. Reason: added Motorolla |
16th Jun 2017, 8:37 pm | #63 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
This has been a great thread with lots of useful comments and links. Can I add one more? It is by LZ1AQ who has clearly done a lot of theoretical and practical work on the subject. He also offers assembled pcbs. Here it is
http://www.lz1aq.signacor.com/docs/w...op-antenna.htm I definitely want to make one but feel rather spoilt for choice and can't make my mind up which approach to go for! Ian |
16th Jun 2017, 9:57 pm | #64 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
That's a rather good article.
The use of a grounded-base input stage is appropriate to the source impedance presented by a small loop. Staying balanced right up to an output transformer and breaking any common mode path looks good too. 2N2222As are beter than usually thought. Yep, I like this design. Wellbrook's use of transformer feedback may be a bit better, but it may be a bit light on gain. Stockton's first law: "The cost of making a decision is inversely proportional to the relative benefit of making the right decision" in other words the time taken to decide between, say, two cream buns tends towards infinity as the difference between the buns tends towards zero. So sometimes a perfectly valid way to decide is to toss coins. David
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16th Jun 2017, 11:09 pm | #65 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
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16th Jun 2017, 11:34 pm | #66 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
Hi.
Just a little more progress with the loop aerial circuit board, pic attached. I've made many PCBs in the past but this is my first experience of making a 'Manhattan' style board. It's a little fiddly to affix the 'tiles' to the board. I used superglue in the end rather than Araldite. I marked an outline on the main board to assist when glueing on the tiles which were applied with tweezers. Inevitably, glue finds it's way on the tweezers which need to be kept clean to avoid problems. It's satisfying when it takes shape and with more practice good results should be possible. For the bifilar transformer Gary specifies 27 swg ECW in his article. I wound the transformer using 26 swg which is fractionally thicker and assume will be OK to use. I always remember the standard wire guages were invariably in even number values. These odd values appear to be as a result of conversion from modern metric sizes. I hope to get on with the assembly of the board over the weekend. Regards Symon. |
17th Jun 2017, 1:42 am | #67 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
Regarding the gain on the 2N5109, I have two of the RCA ones which show gains of 44 and 56 as measured on the simple multimeter tester which puts 10uA in to the base with 3V CE (at room temp). However, I'm not too sure just what significance to attach to such tests
B
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17th Jun 2017, 5:17 am | #68 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
Don't attach much significance. At 3v these devices are not really getting into the straight part of their collector characteristic and they are RF power devices. It's a bit like testing KT88s on a valve tester. Good enough to see that a device is basically working, but no-where near showing its performance at its operating point.
David
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17th Jun 2017, 3:33 pm | #69 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
Hi.
For the record, I checked the gain of the five 2N5109 transistors, measured ambient temperature was 22 degrees Centigrade. Base current (Ib) was approximately 100uA. HFE readings were 40, 44, 45, 46 and 82. In Towers International Transistor Selector, the HFE is quoted as 40 (minimum). Typical HFE would be twice the minimum value. So on the surface of it, four of these appear to be on the low end of what's expected. The question is whether the static test base current of 100uA is a realistic enough test for an RF amplifier transistor. My gain tester was built from one of G.A. French's Suggested circuits, Radio Constructor, August 1974. Regards Symon. |
17th Jun 2017, 6:27 pm | #70 | |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
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17th Jun 2017, 7:00 pm | #71 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
Hi.
I think it is mainly the device's good linearity and high transition frequency as David pointed out in post#39. It will be interesting comparing similar RF transistors performance wise in this loop amplifier set up. Regards Symon |
17th Jun 2017, 7:00 pm | #72 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
The ratio of the power a KT88 runs at versus the nearest a valve tester can get is actually less extreme than the ratio of the power a 2N5109 or 2N3866 runs at versus the power a multimeter set to "Hfe" runs a device at.
Under these circumstances the Hfe result will definitely be on the low side. Those 40's sound OK. Good manufacturers stick plots of Hfe versus Ic in their datasheets. It's linearity in general. OIP3, Crossmod, blocking, all of the above. The approach is to use a beefy device over a fraction of its power capability, that the smaller the section of the curve you exercise it over, the straighter it appears. David
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17th Jun 2017, 11:02 pm | #73 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
I have a vague memory that listening stations in WW2 ran the aerial into an 807 before splitting. Not quite a KT88 but not far off.
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18th Jun 2017, 12:56 am | #74 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
I think Pat Hawker mentions that in Tech Topics. It may have been multi-807's to isolate the HRO receivers from each other, as those receivers had a fairly high level of LO leakage from the aerial socket.
I might be confusing two stories though....
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18th Jun 2017, 6:30 am | #75 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
David G4EBT
What is the physical size of the GT PCBs and Boxes please, so that we might get the proportions correct. Thanks, mike |
18th Jun 2017, 8:04 am | #76 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
Large listening stations would use one antenna into a distribution amplifier feeding a number of receivers. These amplifiers later became known as 'multicouplers'
http://www.pdp-11.nl/racal/ma174/startpage.html There were later versions with transistors handling up to 40 receivers. They have to be broadband so all the receivers can wander wherever needed. You'd never get agreement on where to tune a preselector. They have to be very, very linear to prevent intermod, crossmod etc at the levels created by the sum of all signals from a wideband antenna. Back then the HF bands were heaving with signals, some very large. Transmitting sites were kept away from such receiving sites as much as possible. They had to use one antenna input stage followed by individual amplifiers for each receiver feed in order to get enough attenuation of each receiver's LO leakage heading to all the other receivers. Even later sets with much better leakage than the HRO needed this. Ultra low leakage sets only came along with the Tempest requirements. David
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18th Jun 2017, 9:25 am | #77 | |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
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The boxes which Gary specified are from CPC/Farnell in the UK, but doubtless you'll have a supplier of something similar in Oz. The box is described as 'IP55 Polypropylene Enclosure - 90x90x50mm - 220-L' Ridiculously cheap in the UK at £1.57 GBP inc VAT (sales tax): http://cpc.farnell.com/eterna/220-l/...m%2Fw%2Fsearch The smaller enclosure used for the power feed box is from the same supplier, and externally is 89x43x37mm. Described as: IP55 Thermoplastic 10 Entry Junction Box Enclosure with Screw Terminals - 89x43x37mm - 310 408 01 http://cpc.farnell.com/spelsberg/310...p55/dp/EN83938 The datasheet is here: http://www.farnell.com/datasheets/1826133.pdf Ironically, though this box is the smaller of the two, it's more expensive, but still cheap at £2.17 GBP inc VAT. Hope that helps Mike.
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18th Jun 2017, 10:04 am | #78 | ||
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
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As to housing the amp inside the shed with leads to the outside loop, what a good idea - the wire leads will simply be part of the loop. Why didn't I think of that, albeit the amp that Gary gifted to me is fully encapsulated. Incidentally, I'm a little puzzled by all the 'war stories' of fake transistors from the Orient. All the suppliers of 2N5109s that I've seen on e-bay links have done thousands of trades across a wide range of items with no adverse feedback. For years I've bought transistors, ICs, resistors, capacitors, pots, valves, test gear, kits for projects and so on from China, often at prices that seem too good to be true, and have yet to come across anything duff, yet to read some of the comments in this thread, faking is rife, if not endemic. Huh? I must have been very lucky then!
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18th Jun 2017, 10:56 am | #79 |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
A fair amount of luck, I think, David. If you've been buying mostly jellybean parts then there is less profit in faking those so your chances are better. The risk increases once you go looking for premium parts which just happen to be in standard packages. Then someone can scrub off the ID of a cheapie and print something much more expensive in its place.
I got bitten when I worked at HP. A large multinational assembler built a prototype board I'd designed. The AD811 video amps they bought were fakes. A boss blamed me for them not working and never even apologised when the cause came to light. They worked perfectly with the right parts. RF power parts are faked. Parts for medical scanners, radiotherapy machines, and airliner engines have all been found faked. Scary? I wondered about some of the modern whizzy opamps as active antenna amplifiers with feedback controlled linearity and lots of volts swing, but their best noise performance is with much higher source Z than a small loop gives. David
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18th Jun 2017, 4:13 pm | #80 | |
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Re: Magnetic Loop Receiving Aerial (Gary Tempest)
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